The present invention relates to a process for producing a polyvinylidene chloride resin having excellent heat and light resistance. More particularly, the present invention relates to a process for improving the thermal stability and the light stability of a vinylidene chloride resin by adding thereto small amounts of Vitamin E, an alkyl ester of thiodipropionic acid and an alkali metal salt or an alkali earth metal salt of a phosphoric acid.
In general, vinylidene chloride resins comprise a vinylidene chloride copolymer to which suitable quantities of plasticizer and stabilizers are added to produce desired properties such as excellent oxygen barrier, moisture resistance, transparency, flexibility, oil resistance and heat shrinkability. With such properties, vinylidene chloride resins are very desirable as packaging materials for various foods and, recently have come to be used in an increasingly wider range of applications. The major shortcomings of vinylidene chloride resins are that they show a low thermal stability during melt processing and that products molded therefrom have poor heat and light stability.
To improve the thermal stability of vinylidene chloride resins during melt processing, heat stabilizers such as phenols, amines, sulfur or phosphorus compounds have been added. To improve the light stability of molded vinylidene chloride resinous products, light stabilizers such as salol (phenyl salicylate), benzophenone or benzotrizole have been added to the vinylidene chloride resin. However, these stabilizers have shortcomings in terms of amounts required, cost, color, and smell of resultant products, food sanitation and safety. Further, no single stabilizer has been able to meet both the thermal stability and the light stability requirements.
Desiring to improve the thermal stability of vinylidene chloride resins and the light stability of the resultant products molded therefrom, the Applicants undertook a series of intensive studies which took into account the aforementioned drawbacks of prior art stabilizers. In so doing, the Applicants found a novel process which comprises adding to a vinylidene chloride resin, small quantities of stabilizers, according to the present invention, thereby yielding a vinylidene chloride resin substantially free of discoloration, even after exposure to a thermal history of an ordinary melt processing, which in turn results in moldings having good external color and substantial resistance to discoloration either with time or with exposure to light.